Removal of sacrificial cores by electrochemical machining

ABSTRACT

A sacrificial core ( 304 ) defining a cavity in a metallic or non-metallic shaped article of manufacture, e.g., a casting ( 302 ), is made from a metal that can be electrolytically dissolved. The sacrificial core ( 304 ) is removed from the article ( 302 ) by electrochemical machining. The sacrificial core ( 304 ) may be a hollow shell ( 306 ) incorporating an integral electrode ( 308 ) within the shell and electrically insulated from the shell.

This application is a continuation-in-part of Ser. No. 09/200,959 filedNov. 30, 1998 now U.S. Pat. No. 6,221,235.

ORIGIN OF THE INVENTION

The experimental work leading to this invention was funded in part byU.S. Air Force Materials Command Contract No. F33615-97-C-5275.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to removal of cores from near net shaped articlesof manufacture such as castings and more particularly to removal ofsacrificial cores by electrochemical machining.

2. Brief Description of the Prior Art

In the manufacture of articles having complex shapes it is desirable tominimize the amount of machining required to form the final shape.Similarly, in the manufacture of articles from materials that, aredifficult to forge or machine, such as hard and refractory metals,machining steps are to be avoided, if possible. Accordingly, methods ofmanufacture have been developed in which the raw material is formeddirectly into a final shape, or at least a shape that requires littleadditional processing. Such manufacturing has come to be known as “nearnet shape forming”.

For example, in the manufacture of a complex part from a refractorymetal, it is sometimes necessary to provide one or more cavities, e.g.,recesses and/or passages, within the body of the casting. Forming suchcavities by conventional mechanical machining may be difficult,impractical, or even impossible for some refractory materials and/orsome geometrical shapes of the cavity. Accordingly, processes have beendeveloped to produce articles having such internal structures by nearnet processing methods such as investment casting, hot isostaticpressing (HIP), permanent mold casting, pressure casting or squeezecasting, injection molding, sintering, firing of a green preform, andthe like. Typically, a sacrificial core is positioned within a mold inwhich the article is formed or within a preform that is subsequentlyformed to a near net final shape. For example, a core may be positionedwithin a casting mold, and the mold may then be filled with molten metaland cooled to solidify the metal.

After the article has been formed into its near net final shape it isremoved from the shaping structure, e.g., a mold. However, ordinarilythe core must be separately removed. In many cases the shape of the coreor its close fit within the shaped article prevents it from beingremoved in one piece, especially in the case of precision castings orthe like, where the final dimensions of the article are formed in themold. Consequently, the use of sacrificial cores that are removed bydestroying them has been developed.

Ceramic cores have been used in casting refractory metal parts. However,the removal of the hard ceramic must be accomplished by mechanical meansor chemical leaching, both of which are time-consuming and costly.Accordingly, it has been proposed to use metal cores that can be removedby chemical leaching. For example, the use of steel cores has beenproposed for casting articles from refractory alloys of cobalt, nickel,titanium and the like. The steel cores may be removed by chemicalleaching with strong acids such as nitric and hydrochloric acids.However, even with the use of such corrosive materials, removal of thecores by chemical leaching is extremely slow and difficult, especiallyin deep recesses and narrow passages.

Accordingly, a need has continued to exist for a method of removingsacrificial cores from near net shaped articles of manufacture that issimpler and more rapid than methods hitherto employed.

SUMMARY OF THE INVENTION

The difficulties encountered in removing sacrificial cores from near netshaped articles of manufacture have now been alleviated by the method ofthe invention wherein the sacrificial core is made from a metal that canbe electrolytically dissolved. The shaped article with sacrificial coreembedded therein, together with a counterelectrode as a cathode, isimmersed in or flooded with an electrolyte, and an electric current ispassed between the core and the counterelectrode with the core beingmade generally anodic with respect to the counterelectrode, whereby thesacrificial core is electrochemically dissolved.

Accordingly, it is an object of the invention to provide anelectrochemical method for removing a sacrificial core from a near netshaped article of manufacture.

A further object is to provide a method for removing a sacrificial corefrom a shaped article to provide a near net shaped cavity within thearticle.

A further object is to provide a method for electrolytically removing asacrificial core from a casting.

A further object is to provide a sacrificial core for a near net shapedarticle of manufacture such as a casting.

A further object is to provide a sacrificial core having an integralelectrode.

Further objects of the invention will become apparent from thedescription of the invention which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic isometric view of a mold used to prepare a castinghaving a sacrificial core.

FIG. 2 is a side elevational view of the mold of FIG. 1.

FIG. 3 is a side elevational cross sectional view of the mold of FIG. 1along the line 3—3 in FIG. 1.

FIG. 4 is a side elevational cross sectional view of the mold of FIG. 1after a casting has been formed within the mold.

FIG. 5 is a cross-sectional view of a casting as shown in FIG. 4 removedfrom the mold and arranged for removal of the sacrificial core byelectrochemical machining.

FIG. 6 is a cross-sectional view of the casting of FIG. 5 after theelectrochemical machining process has proceeded partway.

FIG. 7 is a cross-sectional view of the casting of FIG. 5 after theelectrochemical machining process has proceeded essentially tocompletion.

FIG. 8 is a top plan view of a casting containing a sacrificial corewith an integral electrode.

FIG. 9 is a side-elevational cross-sectional view of the casting andcore of FIG. 8 taken along the line 9—9 in FIG. 8.

FIG. 10 is a top plan view of a casting containing a sacrificial corewith an integral electrode wherein the core is open at both ends.

FIG. 11 is a side-elevational cross-sectional view of the casting andcore of FIG. 10 taken along the line 11—11 in FIG. 10

FIG. 12 illustrates the waveform of a modulated reverse electric currentused in a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

According to the invention a near net shaped article of manufacture isprepared having a core which is made of a metal that can be dissolved byelectrochemical machining. The near net shaped article of manufacturecan be prepared by any conventional procedure, such as sand casting,permanent mold casting, investment casting, hot isostatic pressing,sintering, pressure or squeeze casting, injection molding, or the like.

Although the process of the invention finds its greatest utility in themanufacture of near net shape articles that require little or nosubsequent mechanical finishing, it is not excluded that it may be usedin the manufacture of articles wherein the cavity itself, or a portionthereof, is to be prepared with near net dimensions, although othersurfaces and parts of the article may be subjected to subsequent shapingby conventional methods of mechanical manufacture.

The process of the invention will be illustrated by the subsequentdiscussion of its application to removal of sacrificial cores fromcastings. However, the application of the process to articles formed byother methods is straightforward and will be understood by the skilledpractitioner instructed by this specification.

In preparing a cast article having a cavity, wherein the cavity is to beformed with near net dimensions, typically a mold is first prepared todefine the outer surface of the object to be cast. A core is then fixedwithin the mold to form a cavity, e.g., a recess or passage, in the bodyof the cast article. The mold is then closed and filled with thematerial to be molded, which is introduced in liquid form. The materialmay be any substance that can be introduced into the mold as a liquidand subsequently hardened. Ordinarily the material is introduced in amolten state and hardened by cooling. Thus, a molten metal or a moltenthermoplastic material can be poured or injected into the mold andallowed to cool to form a solid shaped article. The mold is then openedand the casting containing the sacrificial core is removed. The castingcontaining a sacrificial core to be removed by electrochemical machiningcan be prepared by any conventional casting procedure. Accordingly, sandcasting, permanent mold casting, investment (lost wax) casting, and thelike, can be used to prepare a casting containing one or moresacrificial cores.

The material of the core is selected so that it may be removed byelectrochemical machining without eroding or dissolving the materialthat forms the body of the casting. For example, if the body of thecasting is made of a metal, the material of the sacrificial core may bechosen to have an electrode potential that permits it to be dissolved inan electrochemical machining process at a potential that is less thanthat at which the metal of the casting is dissolved. In other cases, ifthe cast metal becomes passivated under the conditions of theelectrochemical machining, a metal sacrificial core that exhibits lesspassivation under the conditions may be selectively removed byelectrochemical machining. Furthermore, the electrochemical potentialmay be established during electrolytic removal such that the sacrificialcore is removed preferentially compared to the cast part. If the castmaterial is not an electrical conductor, e.g., a ceramic or a natural orsynthetic resin, the metal sacrificial core can evidently be removedelectrolytically without affecting the material of the casting, providedthat an alternative means of electrical contact is made and maintainedwith the core.

The process of the invention is particularly suited for preparing shapedarticles from refractory metals and refractory alloys thereof, such asnickel, cobalt, titanium, and alloys thereof, and the like. For castingsmade of such alloys, the sacrificial core is preferably made from amaterial having a high melting point, such as steel.

In order to remove the sacrificial core, the casting containing such acore is contacted or immersed in or flooded with an electrolyte alongwith a counterelectrode to thereby form an electrochemical machiningcell. The counterelectrode is positioned adjacent to the core to beremoved, and an electric current is passed between the electrodes, withthe core being made generally anodic with respect to thecounterelectrode. The counterelectrode may be advanced, if necessary tomaintain an advantageous geometrical relationship with the core to beremoved. Typically the gap between the core and the counterelectrode iskept small to provide a low-resistance current path. The electrolyte ispumped through the gap in order to flush away the dissolved metal ionsand also to provide cooling.

The sacrificial core may be a solid metal element, in which case thecounterelectrode is generally advanced from an end of the core that isexposed on the surface of the casting into the recess formed by thecore. In such an embodiment of the process, the counterelectrode isgenerally hollow and electrolyte is pumped through the counterelectrodeto the gap and flows out around the sides of the counterelectrode.

However, when a solid core is used only a relatively small end surfaceof the core is exposed to the electrochemical machining processconducted by the counterelectrode. Accordingly, such a procedure isrelatively slow. It is preferred to use a hollow member for thesacrificial core and to insert the counterelectrode within the core toprovide a relatively narrow gap between the counterelectrode and theshell of the core. When the sacrificial core method is used to form athrough passage in a casting, the core may have the shape of acylindrical shell, open at both ends. The counterelectrode may then beinserted along the axis of the cylinder for maximum exposure of the coreto the electrochemical machining process. The counterelectrode can bespaced from the interior wall of the cylindrical shell by means ofstandoff insulators to prevent short circuits. The electrolyte is thenpumped between the shell of the core and the counterelectrode and anelectric current is passed to dissolve the core from the inside. If asacrificial core is used to form a recess in the body of the casting,the core may be a hollow cylinder closed at one end and positioned inthe mold to form the recess. The cathode counterelectrode may then beinserted into the interior of the hollow core and the electrolyte can bepumped through a channel in the electrode. This procedure also providesthat the core is dissolved from the interior outward. It is generallypreferred that a sacrificial core according to the invention shouldcomprise a shell and an insulated interior counterelectrode separatedfrom the shell by a passage for electrolyte flow. However, those skilledin the art will understand that it is only necessary that thesacrificial core should have a shaped body that defines the cavity, thatthe counterelectrode be integral with the core, i.e., mounted on thecore or otherwise fixed thereto, and insulated from the shaped body ofthe core, and that the sacrificial core contain a passage or channel forthe flow of electrolyte between some portions of the shaped body of thecore and the counterelectrode. Once the electrolysis is initiated, thewalls of the electrolyte channel will be progressively eroded, therebyenlarging the channel and removing as much of the core as desired.Similarly, the sacrificial core need not be made entirely of anelectrolytically dissolvable metal. Only enough of the core need be madefrom the electrolytically dissolvable metal to assure that when asubstantial amount of the metal has been electrolytically removed, theremaining portion of the core can be easily removed by conventionalmeans. For example, the core could comprise a matrix of anelectrolytically dissolvable metal with an inert filler or a structurehaving inert segments fastened together with fasteners of anelectrolytically dissolvable metal. It is also not excluded that aplurality of sacrificial cores according to the invention may beemployed to define the shape of a single near net shape cavity within anarticle of manufacture.

If the electrochemical machining proceeds evenly in all directions, itmay be possible to remove the entire sacrificial core by electrochemicalmachining. However, if some portions of the core are completely removedwhile others parts still remain, it may be preferable to remove thefinal portions of the core by a standard chemical leaching procedure.Nevertheless, removing most of the sacrificial core by electrochemicalmachining provides a substantially faster core removal process.Furthermore, if most of a metallic sacrificial core is removed byelectrochemical machining, it may be possible to remove the remainder ofthe core other conventional procedures, e.g., by simple mechanicalextraction.

When a hollow sacrificial core is used, the counter-electrode may beintroduced into the core at the time of the electrochemical machiningprocess, or the core may be manufactured with an integralcounterelectrode which stays in the core through the molding process andis connected afterward to the source of electricity and contacted withthe electrolyte.

The process of the invention will be illustrated by its use in preparinga casting having a cavity therein. However, it will be understood by theskilled practitioner, as indicated above, that the process is not to belimited to such castings, but may be applied to any shaped articlehaving a cavity wherein at least a portion of the cavity is to beprepared with near net dimensions.

Referring now to the figures of the drawings, FIG. 1 shows an isometricview of a schematic mold 100 of the invention having a top portion 102and a bottom portion 104, with a sacrificial core 106 fixed within themold. FIG. 2 shows an elevational view of the mold, and FIG. 3 shows anelevational cross-section of the mold along the line 3—3 in FIG. 1,showing the mold in closed position with the sacrificial core 106 fixedin position within the mold by means of pin 108. The mold 100 isprovided with a pour hole 110 and a vent 112.

FIG. 4 shows the mold 100 after a liquid material, e.g., a refractorymetal alloy, has been poured into the mold and allowed to harden to formthe casting 114, having the sacrificial core 106 embedded within thebody of the casting.

FIGS. 5-7 illustrate the process of the invention using a casting 200containing a sacrificial core 202, such as might be prepared by thecasting procedure outlined above. The casting 200 with core 202 ispositioned in a conventional electrochemical machining apparatus havinga cathode or counterelectrode 204 with a central channel 206 forsupplying an electrolyte to the worksite as indicated by the flow arrows208. The electrochemical machining apparatus will include means forsupporting the casting 200, means for positioning the electrode 204 andadvancing it as the machining process proceeds, and means forcirculating electrolyte to the electrochemical machining gap between thecore 202 and the counterelectrode 204. Such means are conventional inelectrochemical machining apparatus and are not shown. The castingand/or core and the electrode are connected to the poles 218 of a sourceof electric current 216 shown schematically by conductors 220.

The counterelectrode 204 is made generally cathodic with respect to thecore 202. Consequently, as the electrochemical machining processproceeds, the core 202 is dissolved and the electrode 204 is advanced tomaintain the optimum gap between the electrode 204 and the core 202.FIG. 6 shows a cross-section of the casting 200 and core 202 after thecore has been partially removed. The electrochemical machining processhas formed a recess 210 having a wall 212 and a bottom 214 in thecentral portion of the core 202. As the machining process continues, thebottom 214 of the recess 210 approaches the end of the core 202 and thewall 212 of the recess approaches the side of the core 202. When theprocess has proceeded to the extent shown in FIG. 7, the core removal isnearly complete. It may be possible, as indicated above, to completelyremove the core 202 by the electrochemical machining process. However,it is possible to terminate the ECM process and finish removing the coreby a chemical leaching step.

FIGS. 8 and 9 show a top plan view and an elevation cross-section of anembodiment of the invention using a hollow shell-type sacrificial core.A casting and core assembly 300 comprises a casting 302 with asacrificial core 304 embedded therein by a casting process. The core 304comprises a shell 306 with an integral counterelectrode 308 having acentral channel 310 for supplying electrolyte. The electrode ispositioned within the hollow core by means of standoff insulators 312.

FIGS. 10 and 11 show a top plan view and an elevational cross-section ofan embodiment of the invention using a hollow cylindrical sacrificialcore for forming a through passage in the casting. The casting and coreassembly 400 comprises a casting 402 with a sacrificial core 404embedded therein by a casting process. The core 404 comprises acylindrical shell 406 with an integral counterelectrode 408. In thisembodiment the counterelectrode 408 is shown as solid and a channel 410for electrolyte flow is formed by the space between the electrode 408and the shell 406. The counterelectrode 408 is positioned within thehollow core by means of standoff insulators 412.

The electrochemical machining conditions used for removing thesacrificial cores according to the invention are generally conventional.Electrolytes such as aqueous solutions of sodium chloride, sodiumnitrate, sodium sulfate, and mixtures thereof, and the like may be usedin conventional concentrations. Electrolytes containing from about 15%to about 35% sodium nitrate or about 15% to about 30% sodium chloride ormixtures thereof have been found to be suitable for removal of steelcores.

In practicing the process of the invention, the potential appliedbetween the cathode and the sacrificial core in the electrochemical cellis adjusted so that the sacrificial core is dissolved, but the materialcomprising the body of the casting is not dissolved. Such adjustment ofthe voltage can be determined by skilled practitioner from knownelectrode potentials of the materials of the casting and core and/or bystraightforward experimentation.

The voltage applied between the electrode and sacrificial core may bedirect current (DC), pulsed current (PC), or a modulated reversingcurrent provided by a modulated reversing electric field (MREF). If apulsed current, produced by a modulated electric field, is used, thefrequency of the pulses may range from a few Hertz up to about 1000Hertz. The duty cycle of the pulses may range from about 1% to about95%.

In certain cases it may be advantageous to use pulse reverse current,produced by modulated reverse electric fields (MREF). MREF may be usefulin removing sacrificial cores which are made of a passivating alloy suchas nickel alloys and the like. The use of MREF in electrochemicalmachining of passive metals is disclosed in assignee's copending U.S.patent application Ser. No. 09/080,264, filed May 18, 1998, the entiredisclosure of which is incorporated herein by reference.

FIG. 12 shows a modulated reverse electric field waveform suitable foruse in the process of the invention. In FIG. 12, the anodic peak currentis shown as I₁ and the anodic on-time is t₁. Similarly, the cathodicpeak current is shown as I₂ and the cathodic on-time is t₂. Therelaxation time, or off-time is t_(o), and the intermediate time isshown as t_(i). The sum of the anodic on-time, cathodic on-time,relaxation period, and intermediate period (if present) is the period Tof the pulse train (T=t₁+t₂+t_(o)+t_(i)), and the inverse of the periodof the pulse train (1/T) is the frequency (f) of the pulse. The ratio ofthe anodic on-time to the period (t₁/T) is the anodic duty cycle (D₁),and the ratio of the cathodic on-time to the period (t₂/T) is thecathodic duty cycle (D₂) The current density, i.e., current per unitarea of the electrode, during the anodic on-time and cathodic on-time isknown as the anodic peak pulse current density and cathodic peak pulsecurrent density, respectively. The anodic charge (Q₁) is the product ofthe anodic current and the anodic on-time (I₁T₁), while the cathodiccharge (Q₂) is the product of the cathodic current and the cathodicon-time (I₂T₂). The average current (I_(ave)) is the average anodiccurrent (I₁D₁) minus the average cathodic current density (I₂D₂).Accordingly the relationships among the parameters may be represented bythe following equations. $\begin{matrix}{T = {\frac{1}{f} = {t_{1} + t_{2} + t_{o} + t_{i}}}} & (1) \\{D_{1} = \frac{t_{1}}{T}} & (2) \\{D_{2} = \frac{t_{2}}{T}} & (3) \\{\frac{Q_{1}}{Q_{2}} = \frac{I_{1}t_{1}}{I_{2}t_{2}}} & (4) \\{I_{ave} = {{I_{1}D_{1}} - {I_{2}D_{2}}}} & (5) \\{{D_{1} + D_{2}} \leq 1} & (6)\end{matrix}$

The application of modulated reverse current (or electric field) inpulsed current electrochemical machining can alleviate a number of theproblems experienced in both direct current (DC) ECM and pulsed current(PC) ECM. For example, use of MREF may promote a more uniformdissolution of the metal from the core and decrease the amount of metalprecipitated as hydroxide near the cathode.

It has been found that when removing steel sacrificial cores from atitanium alloy (Ti6AL4V) matrix that pulsed current (modulated electricfield, MEF) at peak voltage of about 6.25 volts and a duty cycle ofabout 80%, with a relatively to short on-time, i.e., frequency greaterthan 100 Hertz, and a relatively large gap size of greater than about0.5 mm, using 15% aqueous sodium nitrate as an electrolyte gave the bestresults.

It will be understood by the skilled practitioner that if a pulsedreversing current is used to remove the sacrificial core, theelectrolytically dissolvable metal portion of the core must bemaintained generally, i.e., predominantly, anodic with respect to thecounterelectrode, i.e., the net flow of current must be in a directionto cause a net oxidation and dissolution of the electrolyticallydissolvable metal.

The invention having now been fully described, it should be understoodthat it may be embodied in other specific forms or variations withoutdeparting from its spirit or essential characteristics. Accordingly, theembodiments described above are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than the foregoing description,and all changes which come within the meaning and range of equivalencyof the claims are intended to be embraced therein.

I claim:
 1. A method of making an article of manufacture having a cavitytherein comprising providing a mold having a sacrificial core, at leasta portion of said sacrificial core being made from an electrolyticallydissolvable metal, filling said mold with a hardenable flowablematerial, hardening said flowable material in said mold, whereby a solidarticle of manufacture is formed having said sacrificial core embeddedtherein, removing said solid article of manufacture from said mold,providing a counterelectrode, interposing an electrolyte between saidelectrolytically dissolvable metal and said counterelectrode, passing anelectric current between said electrolytically dissolvable metal andsaid counterelectrode wherein said electrolytically dissolvable metal ismaintained predominantly anodic with respect to said counterelectrode,whereby at least a portion of said electrolytically dissolvable metal isdissolved, and removing any undissolved portion of said sacrificial corefrom said cavity.
 2. The method of claim 1 wherein said electrolyticallydissolvable metal is steel.
 3. The method of claim 1 wherein saidarticle of manufacture is a casting.
 4. The method of claim 3 whereinsaid casting is made from a metal.
 5. The method of claim 4 wherein saidmetal is a refractory alloy, selected from the group consisting ofalloys of nickel, alloys of cobalt and alloys of titanium.